Processes for reducing the concentration of NO.sub.x (a common industrial pollutant) in a NO.sub.x containing gas are well known in the art. These processes generally comprise:
(a) withdrawing the NO.sub.x containing flue gas as portion of the combustion effluent from the combustion of a carbonaceous fuel (fluidized-bed boilers are among the most efficient devices for burning carbonaceous fuels; the NO.sub.x is primarily formed from the oxidation of fuel-bound nitrogen); PA1 (b) injecting ammonia into the NO.sub.x containing flue gas in order to reduce the concentration of NO.sub.x in the flue gas (the reduction of NO.sub.x by ammonia is feasible within a narrow temperature range of approximately 1600.degree. F.-2000.degree. F. with an optimum temperature of about 1785.degree. F.; at higher temperatures, the ammonia is converted to NO.sub.x while at lower temperatures, reduction of NO.sub.x by ammonia is less effective); and PA1 (c) feeding the ammonia treated flue gas to a cyclone in order to separate entrained particulate matter from the ammonia treated flue gas.
See for example U.S. Pat. No. 4,756,890 by Tang et al. and European published patent application No. 176,293 by Cooper et al. U.S. Pat. No. 4,770,863 by Epperly et al. teaches that further reductions of NO.sub.x can be obtained by also injecting various enhancers into the flue gas such as ethylene glycol or sugar. Epperly further teaches that the use of his enhancer also reduces the concentration of unreacted ammonia in his ammonia/enhancer treated gas. Such a reduction in the "ammonia slip" (as it is often referred to in the art) is very advantageous since ammonia is a pollutant in and of itself. There is a need in the industry, however, to reduce the ammonia slip in a NO.sub.x reducing ammonia injection scheme without resorting to the use of an enhancer. It is an object of the present invention to meet this need.